Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc
Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc
复制标题
DOI：
10.1007/bf00310452
复制
发表时间：
1992-11
影响因子：
3.5
通讯作者：
H. Kagami;S. Iizumi;Y. Tainosho;M. Owada
中科院分区：
文献类型：
--
作者：
H. Kagami;S. Iizumi;Y. Tainosho;M. Owada
日本西南部内带广泛分布着白垩纪-古近纪花岗岩和同生火山岩。这种强烈的中长英质岩浆作用被认为发生在欧亚大陆的东缘，在中中新世西南日本弧漂离大陆之前，导致日本海的打开。花岗岩类岩石在放射性年龄、岩相学、锶、钕和氧同位素比值方面表现出区域差异。根据锶、钕同位素比值，花岗岩类岩石可划分为位于构造中线和日本海之间的三个带(南、北、过渡)。北带花岗岩类岩石及其伴生辉长岩具有低的初始锶同位素比值(0.7048~0.7068)和高的εNd值(+3~-2.2)，而南带的花岗岩类岩石和辉长岩具有高的初始锶同位素比值(0.7070~0.7088)和低的初始εNd值(-3.0~-8.0.过渡带的大多数花岗岩具有介于南北带之间的锶和钕同位素比值，尽管有一些重叠。上地壳对岩浆的混染不能解释这种锶、钕同位素的地理变化。相反，这种区域差异归因于成分不同的岩浆来源(可能是上地幔和下地壳)，位于南北两带之下。南北两带新生代火山岩中的上地幔包体和下地壳包体的锶、钕同位素比值支持了这一结论。这些比值与各自带中花岗岩类岩石的比值相似。认为白垩纪以前或白垩纪期间，南带下方存在由陆壳组成的微大陆或岛弧，造成南北带花岗岩类岩石成分的不同来源。过渡带花岗岩的锶、钕同位素组成差异很大，其原因是地壳和上地幔两种不同成分的比例不同。
Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial εNd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial εNd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.